The dark liquid that comes out of the barrel is viscous, reeking of caramel and vanilla. Alexandra Nowell siphons it into a tulip shaped glass, swirls, sniffs and sips.

"Vanilla. Dried fruit."

To the uninitiated, it might look like a wine tasting. And it is, in a sense. It's a barley wine tasting. Nowell is lead brewer at Drake's Brewing Co. in San Leandro. What's in her glass is a barley wine that has been aging in Heaven Hill bourbon barrels since last winter. Nowell had no idea what it would taste like until she opened it that day. She finds that it tastes like a delicious, alcoholic cinnamon raisin cookie.

Since the early 1990s, when Samuel Adams was one of the first commercial breweries to put its beer into used charred oak barrels previously used for Kentucky bourbon, a large number of American craft breweries have begun to age beer in barrels that once held bourbon.

Spirited beer

Tasting like boozy hybrids of spirit and beer, bourbon barrel-aged beers typically have high alcohol levels for beer (upwards of 12 percent), are sold at a premium, and are usually better sipped like Cognac, preferably by a cozy fire.

The Bay Area and its surrounding environs are home to some of the style's best examples.

Each year, Truckee's FiftyFifty Brewing Co. ages its imperial stout, Eclipse, in seven or eight different types of bourbon barrels. You can try, say, an Elijah Craig version, a Four Roses version, or an Evan Williams version.

"We don't necessarily expect people to buy them all," says Todd Ashman, FiftyFifty's head brewer. "But if they can, great."

Older is better

The chief flavor differences, Ashman says, come from the age of the barrels. Older ones impart more nuanced flavors and textures from the wood than younger barrels.

Last December's Eclipse, aged in Elijah Craig (18-year) barrels, has an Armagnac quality to it. That release of barrel-aged Eclipse has gotten so popular that FiftyFifty now sells beer futures, similar to the wine industry: Fans pay half up front a year before, and secure their bottles for the year ahead.

As with Eclipse, most brewers barrel-age strong beers, such as imperial stouts and barley wines, because they stand up best to the wood. Hoppy beers generally don't do well: Hops' flavor can react poorly to the small amounts of oxidation that occurs in a barrel.

One notable exception is Grand Cru from Fort Bragg's North Coast Brewing.

After years of making the popular bourbon barrel-aged Old Rasputin Russian Imperial Stout and Old Stock Cellar Reserve Ale, North Coast tried something different. Grand Cru, first released in 2011, is a bourbon-barrel-aged strong pale ale made with Belgian yeast (the same one used in its La Merle Saison) and agave nectar.

After being aged in bourbon barrels for only six weeks, Grand Cru has a golden orange hue, and is extremely dry and Champagne-like.

Finding these experimental angles on barrel aging seems to be the wave of the future. Arne Johnson, the head brewer at Marin Brewing Co., makes a tangy bourbon barrel-aged version of Old Dipsea barley wine inoculated with brettanomyces, a funky tasting yeast usually used in sour beers.

Offbeat blends

And offbeat blends are the norm at Drake's, which opened a tasting room in San Leandro called the Barrel House last year. Their Cuvee Du Mallard is a surprisingly easy-drinking mixture of brandy barrel-aged stout and bourbon barrel-aged scotch ale.

Nowell has a "let's throw it against the wall and see if it sticks" mentality, and barrel aging is among a wide range of techniques she may blend together to get something she likes. On a recent afternoon, Nowell pulled a taste of imperial black IPA from a bourbon barrel that had been aging since last Christmas. Swirl. Sniff. She made a face.

"That's very phenolic and Band-Aidy," she said. "The hops just didn't lend themselves well." Sip. It tastes better than it smells. "I'd maybe blend it with an imperial stout and see what that tastes like."

She seems eager for the challenge. Bourbon barrel-aged beer is new territory. For both brewer and drinker, that means happy discovery.